Several explosions within the past couple of years -- responsible for killing or maiming more than 50 people -- involved the startup of natural gas-fired equipment. As consultants, we have been involved in the aftermath of several of these incidents, trying to get plants running again. In reviewing the root causes of these events, we’ve concluded there is a huge misunderstanding about how special repairs to gas piping are, and how careful one must be when starting up new equipment.
This article seeks to provide the basics regarding codes that apply, basic techniques that must be followed, and hazards to avoid. This is a special area of practice that must be respected. Natural gas piping installations and repairs are not like any other piping repairs. They can be done safely if simple principles are followed. If these are not followed and not respected, the results can be devastating.
Several codes and standards apply to these kinds of situations, including OSHA 1910, NFPA 54 and equipment standards such as NFPA 85 or NFPA 86.
Two codes that apply to gas equipment startups are OSHA 1910 and NFPA 54. The first, OSHA 1910.147, addresses lockout/tagout (LOTO) of energy sources. A lot has been written about this code, and most people follow it conscientiously, at least on the electrical side. Unfortunately, for gas piping, steam and other things that could be dangerous in a pipe, it is complied with much less often. As consultants in a facility, for instance, we often have found a lock on a disconnect and a gas valve closed but not locked.
Even when people try to perform gas piping lockout/tagout correctly, we find they often do not understand the issues surrounding lubricated plug valves and their need to be sealed to hold properly. In the case of a plug valve, which represents 60 to 80 percent of all natural gas piping system valves, there is a space between the plug and the body. If sealant is not applied annually, as required by code, gas will leak past the plug and body even when they are in the closed position. We find that most plants do not have the knowledge or equipment to seal these and have never sealed them in the life of the valve. Hence, closing or locking out a valve in this condition does not necessarily isolate the energy source.
The second code that applies is NFPA 54, which also is called the National Fuel Gas Code. We have found that many firms have not heard of it and do not understand it. The lack of isolation points such as blanks, blinds and pancakes, and the lack of purge points installed in most industrial plants, supports this viewpoint. There seems to be little forethought given to the actual installation of the gas pipe, how the gas pipe will be put into service, and the equipment started up.
To help illustrate the points outlined in the codes, here are 10 common gas piping and equipment startup hazards as well as suggestions on how to avoid a problem. Of course, these tips and techniques should be incorporated into a comprehensive, documented procedure for natural gas piping purging, piping system design and equipment startups.
Gas Piping Hazard #1: Purge Points
Purge points are pipe nipples installed at strategic locations in the piping system for the purpose of introducing or removing nitrogen and natural gas at various stages of the process. They generally are 1" schedule 80 nipples with natural-gas-rated ball valves on the ends. It is important to select locations or orientations that ensure the purge points are not susceptible to damage from things like vehicle traffic (being run into with a tow motor or scissors lift).
Gas Piping Hazard #2: Isolation Points
You have to be careful that fuel trains are not exposed to excessive pressures that can damage components during pressure testing. This can be done with the use of line blinds. (Valves can be left open or leak through in the closed position, so they should not be used for pressure testing.) Blinds provide positive isolation and eliminate the possibility for damaging devices such as regulators in the fuel train that are not rated for the elevated test pressures. In some cases, blinds help perform pressure testing correctly. Also, in some situations, pressure testing cannot be performed against a valve.
Gas Piping Hazard #3: Piping Support
During repairs, sections of piping may be disconnected to add tees or install blinds. It is important to ensure that adequate pipe supports exist so that sections of pipe will not fall when disconnected. Remember, the closest support may be on the other side of the disconnected joint.
Gas Piping Hazard #4: Gaskets
Even if they appear to be in good shape, NFPA 54 does not allow flange gaskets to be reused. To ensure leak-free joints, new gaskets must be used as well as properly rated bolts for the flanges. Remember too that if you are mating up flanges, it is raised face to raised face, and flat face to flat face.
Gas Piping Hazard #5: Material Specifications
It is important that only properly rated pipe and fittings are used. Validate that reputable suppliers are used, and the material is free from manufacturing and installation defects such as pinholes in cast-iron fittings, misaligned threads on the cast-iron fittings, and the improper grade of pipe.
Gas Piping Hazard #6: Nitrogen
The air we breathe is 78 percent nitrogen, but two full breaths of pure nitrogen can kill you. This inert gas is nothing to fool with.
Make sure everyone understands this hazard, and make sure nitrogen purge points are marked and located in well-ventilated areas. Also, verify pressure ratings of hoses and regulators: Large liquid nitrogen tanks are capable of producing high discharge pressures. When discharging nitrogen, the purge discharge areas need to be monitored. Everyone involved should be trained on the safe handling of nitrogen.
There are several kinds of purges, including trickle purges and slug purges. A slug purge is a good hard blast of nitrogen that is substantial in flow compared to the size of the pipe. In this case, there is not a lot of mixing, and the nitrogen-to-natural-gas interface is not large. If you introduce the nitrogen too slowly, you will get a lot of mixing and make for a large interface. This is not desired.
Make sure you have enough nitrogen. For instance, having a backup cylinder or liquid container is not a bad idea. (Think of it as cheap insurance.) If you run out of nitrogen during a purge or pressure test, it will cost you a lot more than the extra cylinder of nitrogen. Also, you’ll never know how much you will need for chasing leaks. This part of the process throws all calculations and estimates of how much you will need out the window.
You also will need to discuss with your supplier the size of the regulator you will need, which will depend on the extent of the system you intend to work with. If you get the wrong regulator, you will spend a long time waiting for the lines to fill.
Gas Piping Hazard #7: Discharge Locations
Make sure that purge end points where natural gas may be released are outside and at least 25' from any ignition source. Have areas roped off for security to keep ignition sources, including vehicles, and people away.
To bleed a line, never discharge gas into a building. Always have a way to get the gas out of the building. A garden hose works fine. Make sure that the hose is grounded, and that everything is at the same electrical potential.
Gas Piping Hazard #8: Sampling Devices
During natural gas introduction and removal, do not use a combustion flue gas analyzer for sampling and monitoring. Instead, use a good quality, recently calibrated LEL (lower explosive limit) meter. For instance, a four-gas meter can be used to monitor oxygen levels during nitrogen post-repair purging.
Make sure that at least two of each device are available and that they are calibrated. Stopping the process due to instrumentation error can cause a hazard. One meter can be used for barrier protector for the people near the purge end point, and the other meter (used with an at least a 6' long sensing tube) can monitor conditions at the actual discharge point. Don’t actually stand at the discharge point in harm’s way. Instead, perform a timed and measured discharge; then, with the flow stopped, carefully approach to do an LEL check in the end of the purge hose.
Never trust your nose. Gas utilities inject Mercaptan, a sulfur derivative, with a goal to obtain a 1 percent by volume detection threshold for the average person. However, as we age, our ability to detect smells fades. Also, Mercaptan is absorbed by rust, new steel pipe and concrete. Hence, you can never trust the smell as a reliable gauge for when danger is imminent.
Gas Piping Hazard #9: Piping Integrity
During repairs or additions, piping may not be installed properly or disturbed during work. NFPA 54 requires that you have documentation of pressure testing for new or repaired piping systems prior to introducing natural gas. The results of these tests should be retained for the life of the piping system.
Any section of piping that has undergone recent additions or repairs should be evaluated if no test records exist. Our firm has established as a best practice the use of pressure chart recorders. These paper and pen battery-powered recorders come with small pressure increment gradations (e.g., 1 psig) and provide excellent records of pressure and hold times of tests. The codes also call for reviewing a gauge to see no discernible loss. We like to install a gauge for this purpose in parallel with a chart recorder. This gives you some backup and cross verification. Make sure it is at least a 4" gauge. We try to use a 6" face just to error on the side of caution and quality.
Gas Piping Hazard #10: Equipment Startups
There will be a “moment of truth” period where everyone believes that gas is actually at the burner. Remember, the only thing that an LEL meter can tell you at 100 percent indication is that you have reached 4.3 percent natural gas concentration in air. We try to go some specific timed amount past this point just to get higher than this at the burner.
Once you actually try a relight, it is not likely that you will not start on the first try. We do this in a stepped and staged manner to avoid problems. In some cases for extensive gas piping systems, we light a portable burner at the ends of mains in a safe location. This establishes that we have proper gas concentrations at specific locations. In any case, we always shut the main gas off and only try to light pilots first. We also never do more than three light-off attempts without a dry run (no gas) to make extra sure we are clearing out the firebox.
Anytime natural gas piping systems are designed or worked on these potential hazards should be evaluated and addressed. While the process is not simple, it can be completed safely if planned and properly implemented.
Note: This article was originally published with the headline, "10 Mistakes that Could Kill You," in the October 2009 of Process Heating magazine.